1. Field of the Invention
This invention relates to an ultrasonic actuator, such as an ultrasonic motor or the like, having a structure wherein slits are provided on a traveling-wave-forming surface of an elastic member in which a traveling wave is formed.
2. Description of the Related Art
In an ultrasonic motor, a traveling wave is formed in an elastic member made of metal. The elastic member is formed, for example, in the shape of a circle or an ellipse by a piezoelectric element or the like. A surface opposite to the surface on which the piezoelectric element is fixed is in pressure contact with a relative-movement member, and the elastic member and the relative-movement member are driven due to friction by the traveling wave to provide relative movement. Since the amplitude of the traveling wave formed in the elastic member is small, it is arranged in most cases to lower the neutral axis of vibration, for example, for the purpose of increasing torque by providing a number of projections divided by slits on a traveling-wave-forming surface of the elastic member in the circumferential direction.
Such an ultrasonic motor can be applied to various uses. It has been proposed, for example, to use the motor as a driving source for performing reciprocating drive of a print head of a printer.
In such an ultransonic motor, a friction surface is usually fixed on a surface contacting the elastic member of the relative-movement member.
One example of a vibration wave motor is shown in U.S. Pat. No. 4,580,073, which discloses a vibration member having a plurality of projections and being driven by an electro-mechanical conversion element. The projections drive a movable member by receiving travelling vibration waves generated by the conversion element.
When such an ultrasonic motor is used, for example, as a driving source for driving a print head of a printer as described above, the reciprocating stroke for the print head is constant and the print head is moved at a constant pitch. Thus, it happens that the positional relationship between projections formed between slits in the elastic member and the friction surface become identical at the start and stop of the ultrasonic motor. If cases of slight deviations in the positional relationship are included, the possibility of the projections and the friction member stopping in nearly identical positional relationship is high.
Hence, there is the problem that indentations due to the projections on the elastic member are produced at identical positions in the friction member, causing wow/flutter and reducing torque perfomance.
When the ultrasonic motor is used as a driving source for driving a print head as discussed above, additional elements are used to effect movement of the print head. For example, rotation of the rotor, or relative-movement member, is transmitted to a belt through a transmission. The belt is connected to the print head and can move the print head back and forth. In addition, a rotary encoder can be used to monitor movement of the print head.